1. Field of the Invention
The present invention relates to a driving device for driving a plurality of driven members in the same direction or in opposite directions, and particularly to a driving device having a driving element and a driving rod provided for each of driven members so that the driven members can be driven individually.
2. Description of the Prior Art
To drive a plurality of driven members along the same line while keeping them in a predetermined positional relationship, it is customary to drive all of the driven members by means of a single driving element combined with a guide mechanism that guides the driven members along the driving direction while controlling their relative positions. For example, in a zoom lens system, helicoids having different pitches are provided, as a guide mechanism, on a lens barrel, so that, by converting the rotation amount of a motor into different driving amounts for individual movable lenses, the movable lenses are kept in a predetermined positional relationship. This makes it possible to vary the focal length while keeping the focal point fixed.
In recent years, it has been becoming increasingly common to keep driven members in a predetermined positional relationship by providing a driving element and a driving rod for each of a plurality of driven members so that the driven members are driven individually and the outputs of the driving elements are adjusted individually. A driving device based on this principle requires a greater number of driving elements, but does not require a complicated guide mechanism that demands high precision. Accordingly, a driving device of this type can be realized with a simple structure, and is thus particularly suitable for cases where light-weight driven members are driven by small amounts.
As an example of driving device having a driving element and a driving rod provided for each of driven members, FIG. 29 shows a driving device designed for use in a taking lens system of a digital camera. This driving device 16 is for driving two movable lenses LA and LB included in a zoom lens system.
The driving device 16 is composed of two piezoelectric actuators 51 and 52 serving as driving elements, two driving rods 53 and 54, and one guide rod 55. The piezoelectric actuators 51 and 52 are, at their rear-end surface, individually fixed to two base blocks (not shown), and the driving rods 53 and 54 are fixed to the front-end surface of the piezoelectric actuators 51 and 52. The piezoelectric actuators 51 and 52, when a voltage is applied thereto, expands or contracts along the direction connecting their front-end and rear-end surfaces in accordance with the magnitude of the voltage applied. The driving rods 53 and 54 are arranged parallel to each other, and the guide rod 55 is arranged below the mid line between the driving rods 53 and 54 and parallel thereto.
The lenses LA and LB are individually held in lens frames 56 and 57. The lens frames 56 and 57 have projections 56a and 57a formed in their obliquely upper portion, and through these projections 56a and 57a are formed through holes through which the driving rods 53 and 54 are placed. The lens frames 56 and 57 also have projections 56b and 57b formed in their lower portion, and in these projections 56b and 57b are formed grooves that engage with the guide rod 55. In a side surface of the projection 56a of the lens frame 56, an opening is formed through which a portion of the driving rod 53 is exposed, and a plate spring 56c is provided by which the portion of the driving rod 53 exposed through the opening is pressed with an adequate force. By the pressing force of the plate spring 56c, the inner surface of the through hole formed through the projection 56a is kept in slidable contact with the driving rod 53. Although not shown in FIG. 29, the projection 57a of the lens frame 57 has the same structure, so that the inner surface of the through hole formed through the projection 57a is kept in slidable contact with the driving rod 54. The wall surfaces of the grooves formed in the projections 56b and 57b of the lens frames 56 and 57 are kept in loose slidable contact with the guide rod 55 so as to prevent rotation of the lenses LA and LB.
A piezoelectric actuator, when the voltage applied thereto varies abruptly, expands or contracts abruptly and, when the voltage applied thereto varies gradually, expands or contracts gradually. As the piezoelectric actuators 51 and 52 expand or contract, the driving rods 53 and 54 are displaced. The lens frames 56 and 57, which are kept simply in slidable contact with the driving rods 53 and 54, follow the displacement of the driving rods 53 and 54 when the displacement is slow, but cannot follow the displacement and thus remain where they are when the displacement is fast.
Accordingly, by causing an abrupt rise followed by a gradual drop repeatedly in the voltage applied to the piezoelectric actuators 51 and 52, it is possible to drive the lenses LA and LB in one direction; by contrast, by causing a gradual rise followed by an abrupt drop repeatedly in the voltage, it is possible to drive the lenses LA and LB in the opposite direction. The speed at which the lenses LA and LB are driven can be adjusted by varying the magnitude and the cycle of the voltage applied.
By controlling the voltage applied to the piezoelectric actuators 51 and 52 individually and thereby driving the lenses LA and LB individually, it is possible to keep the lenses LA and LB in a predetermined positional relationship.
However, in the above-described driving device 16, in which a driving element and a driving rod are provided for each of driven members, the driving elements, and also the driving rods, are arranged parallel to each other, and therefore the driven members can be driven not through the whole length of the driving rods but through only a portion thereof. This means that the driving rods are unnecessarily long, occupying unduly large spaces.
When piezoelectric actuators are used as driving elements, even though the driving rods are made of a highly rigid material, it is impossible to eliminate elastic deformation completely. For this reason, the longer the driving rods, the more difficult it is to drive the driven members efficiently because of absorption of the driving force from the piezoelectric actuators and delay in transmission of the driving force. Moreover, this leads to loss of energy. Furthermore, the longer the driving rods, the more rigid they need to be. This narrows the choice of the material of the driving rods and increases their cost.
To achieve efficient displacement of the driving rods, the rear-end surfaces of the piezoelectric actuators need to be fixed securely so that their expansion and contraction are transmitted to the driving rods without loss. For this reason, the base blocks, to which the piezoelectric actuators are fixed, are made of stainless steel, which is a heavy, rigid material. However, in the driving device 16 described above, the piezoelectric actuators are fixed to separate base blocks, and this makes the driving device 16 unduly large.
One way to increase the weight of the base blocks without making the driving device larger is, as shown in FIG. 30, by fixing both of the piezoelectric actuators 51 and 52 to a single, integrally-formed based block 60 that is approximately twice as large as one conventional base block. However, in a driving device of this type, the expansion and contraction of one piezoelectric actuators are transmitted, as vibration, to the other piezoelectric actuator through the base block, and this may adversely affect driving. For example, such vibration causes variations in the driving amount among the driven members even if a voltage having a predetermined magnitude is applied for a predetermined length of time with a predetermined cycle.
To achieve accurate driving, it is essential to detect the movement amount or position of driven members. For this purpose, it is customary to provide a graduated member that changes its relative position as driving proceeds and a sensor that reads the graduations marked on the graduated member, so that the position of the driven members is detected on the basis of the number of graduations read by the sensor. One of the graduated member and the sensor is fixed to the driving device itself, and the other is fixed to the driven members so as to move together therewith. The detection of the position by the combination of the graduated member and the sensor is achieved optically, or on the basis of another physical property.
FIG. 31 shows an example of a driving device of this type. This driving device 17 is for driving a lens L, and is composed of a piezoelectric actuator 71, a driving rod 72, a guide rod 73, supporting walls 74 and 75, a magnetized plate 76 serving as a graduated member, and a magnetic resistance (MR) sensor 77. The piezoelectric actuator 71, the driving rod 72, and the guide rod 73 are the same as their counterparts in the driving device 16. The driving rod 72 is slidably supported by the supporting walls 74 and 75, and the guide rod 73 is fixed to the supporting walls 74 and 75.
The magnetized plate 76 is fitted, parallel to the driving rod 72, to a projection 82 formed on a lens frame 81 for holding the lens L. The surface of the magnetized plate 76 is magnetized in such a way that many N-pole and S-pole regions are formed alternately with a predetermined pitch along the direction parallel to the driving rod 72.
The MR sensor 77 changes its electric resistance according to the magnetic field around it. The MR sensor 77 is fixed to a plate spring 78 screwed to the supporting wall 75, and is so arranged as to face the magnetized surface of the magnetized plate 76. The MR sensor 77 has a spacer (not shown) having a uniform thickness bonded over its surface, and is lightly pressed against the magnetized plate 76 by the pressing force of the plate spring 78 so as to be kept at a constant distance from the magnetized surface of the magnetized plate 76.
When the piezoelectric actuator 71 expands and contracts to vibrate the driving rod 72 and thereby move the lens frame 81, the magnetized plate 76 moves together, and the magnetic field around the MR sensor 77, which is fixed, changes periodically. Accordingly, the output of the MR sensor 77 changes periodically. Thus, from the number of cycles of change in the output of the MR sensor 77 and the pitch of the N-pole and S-pole regions of the magnetized plate 76, the driving amount of the lens L is calculated. By accumulating the driving amount from a predetermined reference position, the position of the lens L is determined.
It is also possible to fit the MR sensor 77 to the lens frame 81 to make it movable and fix the magnetized plate 76. Also this arrangement allows determination of the position of the lens L.
In this driving device 17, the position is detected directly by reading the graduations, and therefore it is possible to determine the position of the lens accurately at all time. For example, even when manufacturing errors cause variations in the force by which the driving rod and the lens frame are kept in sliding contact, and accordingly the driving amount by which the lens frame is driven every time the piezoelectric actuator expands or contracts is not constant, the detected lens position includes no error. In other words, highly accurate detection of the position is possible irrespective of the precision of the driving mechanism.
When two or more driven members are driven individually in the same direction or in opposite directions by this method, it is necessary only to provide a driving element such as a piezoelectric actuator, a driving rod, a graduated member, and a sensor for each of the driven members. However, to keep the driven members in a predetermined positional relationship at all times, it is essential to position the individual graduated members accurately relative to each other, and this requires highly accurate assembly.
For example, in a zoom lens system like the above-described one provided with the driving device 16, in which the focal length is varied by driving two lenses, an error in the relative positions of the lenses makes it impossible to obtain a desired focal length, and causes a deviation of the focal point, making it impossible to obtain a sharp image at a fixed position. To prevent this, it is essential to drive the lenses while keeping them accurately in a predetermined positional relationship. This requires accurate positioning of the two graduated members, and accordingly their positioning in assembly requires an unduly long time.
Moreover, providing a graduated member for each of driven members requires a space to be secured for each graduated member, and thus hampers miniaturization of the driving device. This inconvenience occurs even in cases where driven members do not need to be kept in a predetermined positional relationship.